Artificial hip joints have been used and implanted into humans for a long period of time. The joints normally comprise one femoral part which is intended to be inserted or implanted into the femur and which carries a ball-shaped articulation element, normally made of metal or a ceramic material. The joint further normally comprises a cup-shaped member, i.e. an acetabular cup, which is to be inserted into or attached to the acetabulum, and which is to hold a complementary, cup-shaped liner, normally made of Ultra High Molecular Weight Polyethylene (UHMWPE) or an alternative bearing or wear material, in which the ball-shaped element is to articulate or rotate.
Since the depth of the bone tissue which is available in the acetabulum for attaching the acetabular cup is limited, the possibilities of varying the design of the acetabular cup are rather restricted, which poses similar restrictions on the liner. One result of the lack of bone tissue in the acetabulum is that a typical acetabular cup is rather shallow. This fact makes it rather difficult to find a simple and efficient design and method for attaching a liner into an acetabular cup in a firm and secure manner.
The designs and methods used today mostly utilize circumferential beads or spring wires engaging a complementary groove or other snap action means as means for locking a liner in an acetabular cup. These features normally are complemented with anti-rotation means such as anti-rotation lugs etc. preventing the liner from rotating in the cup. In addition, micromovements still occur which may result in wear and debris production.
Some examples of prior designs according to these principles are disclosed in U.S. Pat. Nos. 3,903,549, 4,795,470 and 5,049,158.
These designs have the disadvantage that they are relatively difficult to make and also to use, since separate locking rings or wires as well as relatively large locking beads or grooves might present difficulties when the liner is to be inserted into the cup during an operation in view of the inherent rigidity of the polymer material serving as bearing or wear material in conjunction with the relatively large deformation necessary.
FR-A-2626168 (G. Cremascoli S.P.A.) discloses a modular hip prosthesis including a cup and a polyethylene element comprising a part manufactured in the shape of a spherical skullcap and an slightly conical annular part. The connection between these is strengthened by the presence of a series of indentations formed on the circumference of the cup in planes parallel to the one defining the opening of the cup.
In practice, the conicity of said annular part is said to allow a stable and perfect connection to be maintained between the exterior titanium surface and the interior polyethylene element. The cup is rendered elastic through the provision of a series of radial slits.
We have observed a disadvantage of making the annular part conical in that the titanium ball might cause impingement on the interior polyethylene element. The radial slits are also disadvantageous in that many surfaces are generated during their manufacture. This can cause the generation of wear particles between the elements, resulting in the dispersal of polyethylene particles through organic tissue and possible bone breakdown.